We have always thought grasses, weeds and trees caused those common seasonal allergy symptoms? a new research now shows parasites are actual cause for those sinus woes.

The plants’ harmless pollen enzymes confuse the immune system into triggering the runny nose and watery eyes chain reaction, but humans might have initially developed that defense to fight off parasites.

“So the idea is that the protease allergens (protein-splitting enzymes) cleave the same sensor proteins that evolved to detect proteases produced by the parasitic worms. Thus the same response is induced unintentionally, and this may explain why this class of allergens has allergenic activity,” Discovery News quoted Ruslan Medzhitov, an immunobiologist at Yale University as saying.

Understanding exactly how allergies work has also eluded scientists, as the immune system reacts differently to allergens than other foreign substances. And it was only recently that the researchers identified a type of white blood cells called basophils as being responsible for many allergic responses.

For instance, when the immune system mistakenly tags ragweed pollen as a parasitic invader, basophils alert the immune system to secrete the specialized antibody immunoglobulin E (IgE).

That triggers the release of histamines, or chemicals released by cells of the immune system during the inflammatory response, which, in turn, causes those annoying symptoms of allergies.

“The point about allergies is that you respond with a class of antibodies called IgE whose function remains unknown,” said Marshall Plaut, chief of the allergic mechanisms section at the National Institutes of Allergy and Infectious Diseases .

It takes repeated allergen exposure over time to build up a strong IgE immune response and reduced symptoms, which is why seasonal allergies can seemingly strike out of nowher.